Referring to FIG. 1, there is shown a plan view of a conventional installation for mounting a shadow mask 10 within the glass envelope 18 of a color CRT. Additional details of the resilient metal holders used for attaching the shadow mask 10 to the inner surface of the CRT's glass envelope 18 are shown in FIGS. 2 and 3. FIG. 4 is a partial sectional view of shadow mask 10 shown in closely spaced relation to a display screen, or glass faceplate, 28 attached to the front of the CRT's glass envelope 18.
Shadow mask 10 includes an inner portion 14 having a plurality of spaced electron beam passing apertures, some of which are shown as elements 10a in FIG. 1. Disposed about and connected to the inner apertured portion 14 of shadow mask 10 is a shadow mask skirt 12. The shadow mask's inner apertured portion 14 and outer skirt 12 form a generally planar, or flat, structure. Attached to the outer periphery of the shadow mask skirt 12 by conventional means such as weldments and disposed about the shadow mask skirt is a shadow mask frame 16 having a generally rectangular shape. Disposed about the shadow mask frame 16 and attached to respective upper, lower and lateral portions of the shadow mask frame are four resilient metal holders, or springs, 20a, 20b, 20c and 20d. All four resilient metal holders are identical in configuration, structure and function, with resilient metal holders 20c and 20d respectively shown in FIGS. 2 and 3. Resilient metal holder 20d is described in detail below, with the description provided being equally applicable to the remaining three holders.
A first end of resilient metal holder 20d is securely attached to the shadow mask frame 16 by conventional means such as spot weldments 30. Incorporated in resilient metal holder 20d are a pair of spaced embossed portions 32 which facilitate flexure of an intermediate portion of the metal holder relative to the first end portion attached to the shadow mask frame 16. A second, opposed end of the resilient metal holder 20d is provided with an aperture 34 for receiving a mounting stud attached to an inner surface of the CRT's glass envelope 18. Thus, each of the four resilient metal holders 20a-20d includes a respective aperture for receiving in a tight-fitting manner a respective mounting stud 22a-22d as shown in FIG. 1. Each of the mounting studs 22a-22d is attached to an inner surface of the CRT's glass envelope 18 by means of a respective mounting fixture, as shown for the combination of mounting stud 22d and mounting fixture 24 in FIG. 4. Each combination of a mounting pin and mounting fixture is attached to the inner surface of the CRT's glass envelope 18 by conventional means such as a weldment or glass frit.
As also shown in FIG. 4, shadow mask 10 with its large number of electron beam passing apertures 10a is disposed in closely spaced relation to the CRT's display screen 28 on a forward portion of glass envelope 18. Disposed on an inner surface of the CRT's display screen 28 is a phosphor layer 36 comprised of a large number of discrete phosphor deposits, or elements, which emit light when an electron beam is incident thereon to produce a video image on the display screen.
Shadow mask 10 undergoes thermal deformation as a result of the electron beams which are directed onto the shadow mask and in large part pass through its many apertures 10a. Those portions of the incident electron beams which do not transit the apertures 10a in shadow mask 10 raise the temperature of the shadow mask producing the aforementioned thermal deformation, which is commonly referred to as mask "doming". Because of this heating of the shadow mask 10 and the metal-to-metal contact between the shadow mask and each of the four resilient metal holders, heat is transferred from the shadow mask to each of the four resilient metal holders. Heating of the resilient metal holders causes the metal holders to also undergo thermal deformation causing the four resilient metal holders to rotationally displace the shadow mask 10 in a clockwise direction in an X-Y plane, or in the direction of arrow 26 in FIG. 1. Clockwise rotation of the shadow mask 10 is due to the axis of deformation of the four metal holders which is along their respective longitudinal axes.
Resilient metal holders of the bi-metal type are typically used in combination with a shadow mask comprised of aluminum killed (AK) steel. These bimetallic type resilient metal holders compensate somewhat for shadow mask rotation by moving the shadow mask 10 and frame 16 assembly toward the CRT's display screen, or outwardly from the plane of FIG. 1 in the direction of the Z-axis of the CRT. In the case of a shadow mask comprised of Invar which is typically flatter in shape and includes a finer aperture pitch, i.e., increased number of apertures begin per unit area, resilient metal holders comprised of a single metal such as of a stainless steel (either SS302 or SS310) are used. However, both prior art approaches suffer from the aforementioned directional rotation of the shadow mask in the X-Y plane due to the heating and thermal deformation of both the shadow mask and the resilient metal holders. Shadow mask rotation degrades color purity of the video image presented on the CRT's display screen, particularly in the corners of the display screen, because of the associated electron beam landing misregistration. The problems of electron beam landing misregistration and color purity degradation become even more severe in the case of shadow masks with finer aperture pitch and flatter configurations as commonly encountered in larger CRT's.
The present invention addresses the aforementioned limitations of the prior art by providing a shadow mask mounting arrangement including a plurality of spaced, resilient metal holders for attaching the shadow mask to the inner surface of the CRT's glass envelope in a manner which maintains alignment of the mask's apertures with phosphor deposits on the CRT's display screen. The inventive resilient metal holders maintain this alignment even as the shadow mask and holders undergo thermal deformation during CRT operation.